In DSL technology, communications over the local subscriber loop between the central office and the subscriber premises is accomplished by modulating the data to be transmitted into a multiplicity of discrete frequency carriers which are summed together and then transmitted over the subscriber loop. Individually, the carriers form discrete, non-overlapping communication subchannels of limited bandwidth; collectively, they form what is effectively a broadband communications channel. At the receiver end, the carriers are demodulated and the data recovered.
The data symbols that are transmitted over each subchannel carry a number of bits that may vary from subchannel to subchannel, depending on the signal-to-noise ratio (SNR) of the subchannel. The number of bits that can be accommodated under specified communication conditions is known as the “bit allocation” of the subchannel, and is determined for each subchannel in a known manner as a function of the measured SNR of the subchannel and the bit error rate associated with it.
The SNR of the respective subchannels is determined by transmitting a reference signal over the various subchannels and measuring the SNR's of the received signals. The loading information is typically determined at the receiver, or “local” end of the subscriber line, e.g., at the subscriber premises, in the case of transmission from the central office to the subscriber, and at the central office in the case of transmission from the subscriber premises to the central office, and is communicated to the other “transmitting,” or remote end, so that each transmitter-receiver pair in communication with each other uses the same information for communication. The bit allocation information is stored at both ends of the communication pair link for use in defining the number of bits to be used on the respective subchannels in transmitting data to a particular receiver. Other subchannel parameters such as subchannel gains, time and frequency domain equalizer coefficients and other characteristics may also be stored to aid in defining the subchannel.
Information may, of course, be transmitted in either direction over the subscriber line. For many applications, such as the delivery of video, Internet services, etc., to a subscriber, the required bandwidth from the central office to the subscriber is many times that of the required bandwidth from the subscriber to the central office. One recently developed service providing such a capability is based on discrete multitone asymmetric digital subscriber line (DTM ADSL) technology. In one form of this service, up to 256 subchannels, each of 4312.5 Hz bandwidth, are devoted to downstream, from central office to subscriber premises, communications, while up to 32 subchannels, each also of 4312.5 Hz bandwidth, provide upstream, from subscriber premises to central office, communications. Communication is by way of frames of data and control information. In a presently used form of ADSL communication, 68 data frames and one syncronization frame form a superframe that is repeated throughout the transmission. The data frames carry the data that is to be transmitted and the syncronization frame, or sync frame, provides a known bit sequence that is used to syncronize the transmitting and receiving modems and that also facilitates determination of transmission subchannel characteristics such as signal-to-noise ration (SNR), and the like.
In providing upstream and downstream channels, ADSL modems divide the available bandwidth of the subscriber loop in one of two ways, frequency-division multiplexing (FDM) or echo cancellation. Frequency division multiplexing assigns one set of subcarriers for upstream data and a different set of subcarriers for downstream data. The downstream path is then divided by time-division multiplexing into one or more high-speed channels and one or more low-speed channels. The upstream path is also multiplexed into corresponding low-speed channels.
Echo cancellation assigns the upstream band to overlap the downstream, and separates the two by means of local echo cancellation, a technique well known in V.32 and V.34 modems. With either technique, ADSL splits off a 4 kHz region for basic telephone service at the DC end of the band.